The manufacture of microelectronic devices requires various polymers to be used as a temporary photoresist (“resist”) mask during a photolithography process. These polymers may comprise a range of chemistries to include novolac, polyhydroxystyrene, acrylic, silicone, epoxy, and mixtures thereof, but most commonly the novolac resin that is common to the support of positive acting lithographic systems. After establishing the resist mask, further processing is carried out on an inorganic substrate (e.g. glass, silicon, silicon dioxide, aluminum, copper) as in the case of a thin film transistor (TFT), liquid crystal display (LCD), or the substrate may exist as a thin organic film (e.g. polyimide, polyester) as in the case of manufacturing organic light emitting diode (OLED) devices, collectively and for purposes of this invention, these units are referred to as flat panel displays (FPD). The same or similar practices occur in the manufacture of semiconductor and microelectromechanical (MEMS) devices. Upon completion, the resist mask must be stripped (removed) without effect to the adjacent metals, dielectric, or underlying substrate. It is a desire to provide compositions that comprise metal-safe and solids-borne materials and the associated methods that are proven effective to remove a resist mask and its residue from substrates during electronics manufacturing.
Typical lithographic processes use resist that is exposed to actinic radiation through a photomask, and following its development, rinsing, and drying, the positive or negative acting tone of this resist will produce a corresponding pattern. This pattern provides the design for specific inorganic and organic matter to be etched away (removed) from or deposited (added) onto the substrate. After the process is complete, the resist pattern must be removed, leaving behind the specific pattern design that is spread across the substrate in a layer-like manner. This layering effect is repeated multiple times, comprising different designs and materials, until the final layered device is complete. Of specific importance to this process is the efficient and reliable removal of photoresist and other polymers following each layering practice during electronics manufacturing.
Positive photoresists are commonly composed of novolac or polyhydroxystyrene (Phost) resins and represent the largest volume portion of photoresists produced globally. Applications in microelectronic manufacturing include substrate etching and electrodeposition. These processes commonly require exposing the substrate surface to acidic mineral acids (e.g. sulfuric, hydrochloric, phosphoric, and hydrofluoric) and mixtures of these acids with oxidants (e.g. peroxide, nitric). To achieve the necessary chemical resistance, the photoresist must be baked at elevated temperatures to condense its polymeric framework and oxidize the top skin of its coating, and produce a resistant framework to acidic chemistries in electroplating processes and oxidizing chemistries used in etching. Once this wet-etch or deposition process is complete, the highly baked photoresist must be effectively removed such that no residue remains to allow subsequent processes to proceed without special treatments. It is a desire in electronics manufacturing to effectively remove high-baked photoresist following wet chemical processing without additional special steps.
Positive-tone resists are also used as imaging masks in processes which use plasma-based etching (e.g. dry etching). Lithography practices to produce the resist mask are conducted similarly as is done for wet-etch processing, however, the substrate is present in a reduced vacuum plasma chamber. In this chamber, the substrate lies in a polarized charged radio-frequency condition as established by the chamber operation, whereby upon introduction of gaseous species, the gases become ionized and are attracted to the charged condition of the substrate. The electronics substrate, albeit inorganic, organic, etc., may have metallic layers pre-deposited upon its surface and over that is present the resist mask pattern. Within the vacuum chamber and using specific control of the gaseous mixture (e.g. boron trifluoride, oxygen, argon, etc.), ionized species are formed and bear down onto the substrate to react and produce gaseous by-products (e.g. silicon-fluoride, etc.) that are pulled away by the low vacuum conditions of the tool and captured in a cold trap. During the etching, plasma exposure to the resist mask can exceed 150 degrees C. The plasma impact onto the resist generates a cloud of carbon containing compounds, which redeposit along with other by-products of substrate attack. This redeposit occurs within the active zone of the etching to produce a resistant layer of material along the dimensional wall or along-side the point of contact to the substrate, producing a penetrating geometry via or trench with the redeposit along its wall (side-wall polymer).
The presence of this side-wall polymer is common for all plasma etch processes. This residue is chemically composed of by-products of the plasma, substrate, and organic constituents of resist, for example, silicon, oxygen, carbon, gallium, arsenic, boron, phosphate, titanium, tantalum, tungsten, copper, nickel, aluminum, chromium, fluorine, chlorine, and others. Etch residue can be referred to as an organometallic mixture comprising inorganic species that are bound by carbon-containing material. As described earlier, removal of the resist mask and its residue is required for successful microelectronic manufacturing. Resist mask and residue removal must effectively remove such material following plasma processing.
Successful chemistries for removing side-wall polymer and other resist etch residue must directly interact with the inorganic ions by leaching, suspending, and complexing while the binding effects of the organic material is removed. These cleaners are commonly composed of organic solvents, amines, water, reducing agents, chelating agents, corrosion inhibitors, and surfactants. U.S. Pat. No. 5,496,491 (1995), Ward et al., and U.S. Pat. No. 5,911,835, Lee et al., presents stripping compositions that comprise polar solvents, amines, and inhibitors, as well as hydroxylamine and ethylenediamine tetraacetic acid (EDTA) performing upon micron sized aluminum topographies resulting from plasma processing of microelectronic devices. The reducing agent, hydroxylamine, has been cited extensively in the literature offering selective resist and residue stripping while protection of underlying metal features. Regardless of the choice in the chemistries cited by these inventions and others, the common manufacturing practice involves the management and delivery of large volumes of organic solvent used as the stripper. Aqueous-based compositions of matter which remove resist masking and residue are attractive to the fab. It is beneficial to conduct microelectronic manufacturing in a manner that discourages the use of organic solvents, minimizes safety risk to workers, reduces costly permitting and tool design, and minimizes impact to the environment from wastes.
While it is critical to exhibit good performance in resist stripping within a rapid time frame, it should be understood that these chemistries must be compatible with the design and practice in the tool, as well as the materials of construction. In a common cleaning tool that is designed for resist stripping on large FPD substrates, the parts travel on a conveyor from one chamber to another, from resist stripping, to DI water rinsing, and completing the process with a drying step. During the course of stripping in an FPD processing tool, there normally exists at least two (2) product tanks, and customarily three (3) tanks, that are separate and distinct and arranged in-line with the flow direction of the parts. Substrates entering the tool will be first “washed” by the chemistry in the first tank. The chemistry is sprayed onto the substrate surface, and upon reacting with the resist by swelling, lifting-off, and dissolution from the substrate, it is collected and returned to the tank where it is mixed with existing bulk chemistry, heated and filtered to remove any suspended and undissolved materials, and cycled back to the spray chamber where the process is repeated, whereby this practice proceeds in a continuous manner. To this end, if a company is to consider a new cleaning chemistry, it is always the desire to not change their tooling for reasons of process control, cost, and most important, engineering investment. Should benefits exist which suggest a new chemistry, it is preferred to use the same tool and simply tune the process to meet the performance requirements. It is therefore preferred to conduct new candidate chemistries which remove resist and residue by processing within the same existing tool used at the customer site.
Consistent with the need of the FPD tool that contains confined chambers that support a continuous process, the desired compositions of matter shall be of the low-foaming variety and exhibit a high bath life. The characteristic of foam generation is common for many industrial-grade detergents, for example, in car washes and pavement steam washers, where the presence of foam is preferred as a visual indicator for the presence of soap with a subsequent knee-jerk requirement for rinsing. However, foam is not acceptable for precision parts cleaning, and in some cases will produce a disaster condition. The phenomena of foam is described as the agglomeration of millions of air bubbles which act as a barrier to chemical interaction with the soil and impede its removal. As foam is generated with spray agitation and the moving parts of the tool, it migrates beyond the chamber to other areas outside the tool, and in worse case conditions will spread to the production area (fab) and collect in pools where it produces an unsafe condition for workers, contamination source, and a medium for electrical shock with low-lying high voltage wiring from adjacent equipment. When this occurs, the cleaning tool must be shut down and serviced. With this in mind, formulations must maintain low surface tension for good wetting yet minimize foam, a challenge for a chemist as the occurrence of foam is promoted by driving down surface tension. Whenever operating a chemistry in the fab, it is always an objective to minimize foam production to a level deemed acceptable for use in the tool.
Selectivity in any resist removal process cannot be over emphasized. Namely, as more aggressive chemistries are formulated to achieve a desired performance, this practice must be met without damage to sensitive metals and the underlying substrate. This is especially challenging for resist removal as the reactive agent of choice are alkaline. When using aggressive alkalis, for example alkali hydroxides, they cannot be used without the aid of inhibitors. When these materials are used alone, they raise the pH of the system, cause galvanic corrosion to adjacent metals, while destroying the substrate. Of particular concern in electronics manufacturing is the use of aluminum or alloys thereof. In fact, the aluminum metal that is present in such microelectronic configurations may survive the resist and residue removal step but begin to corrode during rinsing. This is explained by the momentary mixing between the resist stripper and DI water such that the stripper chemistry is diluted until all that remains on the substrate surface is DI water. Depending upon the tool configuration, spray performance, and the absence of any pooling, eddy currents or other irregularities, the complete rinsing may take only seconds. As with most manufacturing processes, irregularities become the norm which may stretch the completion of rinsing and subsequently expose the substrate to diluted stripper chemistry, whereby the pH remains high while a corrosion inhibitor may be diluted to non-reactive state. In this situation, galvanic corrosion of a microscopic region of ultra-sensitive aluminum or its alloy will occur swiftly and present itself during inspection by a scanning electron microscope (SEM) image as a range of conditions from the most serious as the absence of the feature due to it being completely etched (removed) to an intact feature that is stained (blackened). Some companies have attempted to avoid this rinse phenomena by the addition of an interim rinse with isopropanol (IPA), however, this practice is both expensive and a flammability hazard. There is a need, accordingly, for improved stripping compositions which will remove the processed resist in a rapid manner during rinsing with DI water, and preventing the corroding, gouging, dissolving, dulling, or otherwise marring of the microelectronic features.
Many choices exist for aluminum inhibitors, for example, catechol is added to resist stripper chemistries as disclosed in U.S. Pat. Nos. 5,482,566, 5,279,771, 5,381,807, 5,334,332, 5,709,756, 5,707,947, and 5,419,779 and in WO 9800244. However, catechol exhibits certain health and safety compliance issues, and more specifically, catechol is cited as a suspected carcinogen. Several initiatives are active in the electronics industry to discourage the use of hazardous materials and any items that produce unwanted measurable or perceived health risk. The toxicity of catechol, substituted catechol, and pyrogallols, all exhibit a regulated toxicity level that is deemed unacceptable for electronic manufacturing. A key desire is to provide compositions of matter that are safe and exhibit low risk to workers in an electronic manufacturing fab.
It is further well known that silicates offer good protection to aluminum and its alloys in alkaline conditions. This is a common practice in the industrial detergent industry, however, these products typically contain alkaline earth metals such as sodium and potassium, both of which are known contaminant ions for microelectronics. The industry has published guidelines for defining the purity of raw materials in terms of the presence or absence of alkali elements. Non-alkali element silicates (e.g. alkyl silicates) are used in resist strippers as disclosed in JP 1120552 and U.S. Pat. No. 4,628,023, however, these materials exhibit limited solubility. In fact, all silicates have a critical dependence upon solubility, as exemplified in sol-gel technology whereby gels and pourable solutions may be produced from silicates, simply through pH control and the ratio of a semi-aqueous mixture. Given this background, silicates become a good choice for an inhibitor, however, premature deposition and residue must be prevented. It is a further desire to provide compositions of matter that are infinitely soluble in DI water and will not produce a condition of redeposit or residue while being used in electronics manufacturing.
Additionally and most important, there is continued loading of the stripper chemistry with the organic substance, causing a reduction in bath life and if not given proper attention, will reach a condition whereby the activity of the chemistry is exceeded, performance is not achieved within the defined process time and an observation of residue occurs on the surface of the substrate. Bath life may be defined in a number of ways, however, most common to fabs is the number of parts cleaned per unit volume of chemistry for a specific process condition. While cleanliness of the parts is observed as complete dissolution and removal of polymer within the time governed by the process condition, this practice of dissolution can be explained by fundamental laws in chemistry and physics whereby molecular mobility leading to dissolution is directly influenced by temperature and agitation. Higher temperature and agitation leads to higher molecular mobility, minimizing aggregates and precipitates forming, achieving a greater amount of polymer dissolving, and ultimately an increase in bath life. To increase bath life, it may seem a simple act to increase temperature and agitation, however, it becomes impractical due to limitations in operating the tool (e.g. frequency of parts movement, etc.) and the chemistry which most fabs use industrial policy limit of 10 degrees C. above the flashpoint (FP) of the liquid (SEMI S3-91, Safety Guidelines for Heated Chemical Baths). Most important, the risk of damage should be discouraged by the increased frequency of parts movement and corrosion associated with operating at higher temperature. It is further a desire to provide compositions of matter and methods to achieve increased bath life in electronics manufacturing.
Due to the reduced cost structures available, the manufacture of virtually any electronic device is most competitive when it is conducted in Asia. It is understood that the easiest way to reduce manufacturing costs are by increasing the bath life of the cleaning chemistry. Increasing bath life reduces the time intervals to switch-out used with new chemistries, the need for solution heating, and other maintenance practices. Using compositions of matter of a concentrated state, and more specifically, the compositions of matter existing in a concentrated solid state, bath life is increased by replenishing the active agent as it is consumed. In the case of adding solid form active compositions of matter, a relatively small amount of concentrate is added, little or no adjustment is required to the volume, and the process temperature is constant. In the case of a FPD manufacturing line, typical replenishment occurs only for tank #1 (i.e. the “dirty tank”), while the follow-on tank #2 and tank #3 (e.g. for 3 tanks) remain in their existing condition as a level of process assurance. With the simple attention being given only to tank #1, a bath life increase for the entire process is observed to be up to three (3) times that of a normal organic solvent system. It should be noted here that organic solvent systems do not inherently have solid-form concentrated species available to conduct replenishment practices to realize a significant increase in bath life. For maximum flexibility in electronics manufacturing, it is a desire to provide solid-form compositions of matter and methods to increase bath life by simple replenishment directed to the tank chemistry.
As alluded earlier in this invention description, FPD manufacturing requires large volumes of organic solvent chemistry to remove resist and residue from the substrate surface. At the time of this writing, estimates for organic solvent usage for this application are measured in units of metric tons, exceeding several hundred thousand metric tons per year. It is further estimated that 70-80% of this amount is recycled with the remainder becoming a hazardous waste that must be incinerated or buried. The use of organic solvents requires material storage at the fab and a shipping system that cycle the waste off-site to a recycler and return it to the point of use, every step requiring inspection and analysis. Alternatively, this invention as present in a solid-form concentrate may be stored onsite at the fab location, yet is expected to occupy a fraction of storage space due to expected mix practices in DI water to vary between 2-5%. Most importantly, the invention's waste may be treated onsite at the fab location using pH neutralization and filtration technology similar to that practiced for resist developer. In fact, the invention waste may be co-mingled (mixed) with the developer waste. To this end, no storage, shipment, recycling, testing, etc., is required for the invention waste management. Solid born materials are desirable to minimize storage space and waste management practices as compared to conventional activities associated with organic solvents.
Taking these challenges together, there is a pressing need to provide a consistent and universal process, which uses compositions of matter that vary depending upon the performance needs of the unique polymer or residue to be removed, which provides high performance, high throughput, a green process, all at a reduced cost of ownership. Now, therefore, these needs must be met by a material that achieves high performance character within a rapid time frame and operates within a conveyorized spray tool designed for processing large substrates that are typical of the variety found in manufacturing FPDs. The invention outlined in this document provides aqueous-based and solids born compositions that meet the desires stated here as well as discourages the use of organic solvents and is a green product. The invention is a drop-in replacement for organic solvents and meets foam controls and metal safety for the manufacturing of FPDs. Because of the solid form nature of the product, it is simple diluted with DI water and replenishes the bath to provide an increase in useful life of the chemistry to a level whereby the raw-material cost of organic solvents becomes a significant cost. These and other benefits provided by this invention and the variations which may be practiced by those in the art, all provide novelty and improvement in the removal of resist and their residues. Such inventions are considered to be valuable for the processing of FPDs.